Digitally modulated signals are equalized to minimize the effect of non-ideal phase and frequency characteristics of a transmitter, a receiver, and the communication channel between the transmitter and the receiver. Non-ideal channel characteristics produce different phase delays and attenuations for different signal frequency components. In addition to equalization, timing and sampling rate adjustments are also performed in a digital receiver. A pilot tone or other known signal may be used at the receiver to recover and track the sampling rate of the signal transmitter. For simplicity, the term “pilot tone” or “pilot” is used hereinafter to include any signal used to recover and/or track the sampling rate. Information about misalignment of the pilot tone may be used in a phase-locked loop to “lock” the sampling rate of the receiver's analog-to-digital converter to the timing of the transmitter.
In a Discrete MultiTone (DMT) system, the receiver may employ a frequency domain equalizer (FEQ) to compensate for the attenuation and phase distortion caused by the communications channel. Although this is the proper function of an equalizer, the frequency domain equalizer may also try to “compensate” the phase and magnitude of the received pilot, which may negatively impact the accuracy of the sampling rate recovery and tracking mechanism. One way to avoid this problem is to perform equalization and sampling rate recovery at different times, only using the pilot during periods when the equalizer is “off” for timing control. This approach, however, permits the sampling rate the opportunity to deviate unacceptably during the time period when the frequency domain equalizer is “on,” and the pilot signal is not being used for timing control.
Another complicating factor is that the behavior of the sampling rate recovery control loop depends upon the amplitude of the control signal. The filter coefficients in the control loop filter, for example, typically are optimized for a certain loop gain and control signal amplitude in order to provide the desired transient response and sensitivity to noise. Therefore, training of the pilot's frequency domain equalizer coefficient(s) to achieve a normalized amplitude may also be important. This is also true if the pilot frequency is used for symbol synchronization.
The present invention provides a method and apparatus for simultaneous equalizer updating and sampling rate control that also resolves the problems identified above. A known signal, such as a pilot tone, is received and separated into its real and imaginary components. Separate mechanisms are used to process the real and imaginary components in an equalizer. One of the real and imaginary components, preferably the real component, is used to control sampling rate timing in the receiver. The imaginary component of the known signal is used in an equalization adaptation procedure, e.g., updating an equalizer coefficient value. However, the roles of the real and imaginary components may be reversed.
In one non-limiting, example embodiment, the real component of the known signal is set to zero for purposes of equalization. The unequalized, real component is used to recover/track sampling rate timing. Because the real component has not been equalized, the timing recovery/tracking is de-coupled from equalization. Only the imaginary component is equalized and used to update the equalizer coefficient value. Thereafter, the unequalized real component and the equalized imaginary component are combined.
In another non-limiting, example embodiment, a real-valued equalizer coefficient is determined for the known signal. Both real and imaginary components of the received signal are equalized with the real-valued equalizer coefficient. The so-equalized real component has a normalized amplitude for the timing control signal which is desirable in certain timing control applications. The equalized real and imaginary components are combined. The equalizer coefficient is updated using only the imaginary component of the known signal.
A receiver in accordance with the first example embodiment of the present invention includes an analog-to-digital converter for sampling the known signal, and an equalizer for equalizing the sampled signal except for a real component of the known signal. A timing control unit controls the sampling rate of the analog-to-digital converter using the unequalized, real component of the known signal. A receiver in accordance with the second example embodiment also includes an analog-to-digital converter, an equalizer, and a timing control unit. However, a real-valued equalizer coefficient is employed by the equalizer to update both real and imaginary components of the known signal. The so-equalized real component of the known signal controls the timing unit. However, the real-valued equalizer coefficient is updated using only the imaginary component and not the real component of the known signal.